Humans cause 95 percent of all wildfires in two-thirds of the state

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The Perry Fire burns just above a home in the Palomino Valley, north of Reno, Nev., Tuesday, July 31, 2018. The wildfire has consumed 68 square miles of high desert rangeland and forced evacuations north of Reno. Authorities arrested David Radonski, 34, of Reno, on Tuesday as an arson suspect in the fire that started four days earlier near Pyramid Lake. (Jason Bean/The Reno Gazette-Journal via AP)

While weather is driving the severity of California’s wildfires, new research shows that people ignite a lot more fires than nature does.

And we’re starting fires where and when nature normally doesn’t — at places and times where lightning rarely strikes.

But there’s hope: Because we cause them, we can stop them.

“In most of California, if we could stop ignition during extremely high winds and drought and heat spells, like now, that will be an effective approach” to reduce risk, said Jon Keeley, a research scientist with the U.S. Geological Survey’s Western Ecological Research Center in Sequoia National Park, who studied fire patterns since 1910.

These findings come at a time of growing alarm over the state’s 17 major fires. While investigations are still underway, every fire now burning in California may be human-caused; no lightning storms were reported.

The Carr Fire was reportedly caused when a trailer tire went flat and its rim scraped the asphalt, creating sparks. At Southern California’s Holy Fire, a 51-year-old man has been arrested and charged with starting the blaze after a profanity-laden text threatening: “Its all gonna burn.” The cause of the Mendocino Complex fire has not yet been determined.

Keeley’s new analysis found that humans caused 95 percent of all wildfires throughout most of the state since 1910. The rest were caused by lightning strikes, mostly in the rural northeastern corner of the state.

In the Bay Area, the portion of human-caused fires ranged from 93 percent in Contra Costa and Santa Clara counties to 98 percent and 100 percent in Alameda and Marin counties, Keeley reports in an upcoming issue of the International Journal of Wildland Fire.

Human-sparked blazes are expanding the fire season, according to Jennifer Balch, director of the Earth Lab at University of Colorado, Boulder, writing in the Proceedings of the National Academy of Sciences. More than three-quarters of lightning fires started in the summer; in contrast, human-caused fires occurred throughout the year – especially the tinder-dry fall, when winds are high.

We’re also starting fires in places that don’t typically see a lot of lightning, such as the Bay Area and central California. And they’re occurring during the most severe fire weather conditions, she said. For instance, ignition during Santa Ana or Diablo wind conditions would ordinarily be rare; now, it’s commonplace, she found.

“Human ignitions have expanded the ‘fire niche,’” she wrote.

Forrest Gordon Clark, 51, was arrested late Tuesday and is in custody at the Orange County Intake Release Center. He was arrested on suspicion of setting the Holy fire and making criminal threats. (Courtesy of OCSD)

Once started, today’s wildfires are hotter and more destructive than ever. In the western U.S., regional temperatures have increased by almost 2°F since the 1970s, snowmelt is occurring a month earlier in some places, and the length of fire season has increased by almost three months, said Balch.

Climate change accounted for more than half of the increase in aridity of western U.S. forests between 1979 and 2015, according to a paper by Cupertino native John Abatzoglou, now an earth scientist at University of Idaho.

Unusual warmth causes vegetation to dry faster — and fire has to do less work to ignite the adjacent vegetation, so fire spreads faster, according to Abatzoglou. Warmth also causes an earlier snowmelt in the spring, leading to a drier summer.

Furthermore, there’s been an explosion of development in areas at high risk of fires. We start twice as many wildfires on July 4 as any other day.

“The patterns are quite striking, with humans starting fires where people live — and also where lightning is very very rare,” said Abatzoglou. “The story is very different as you move into the Sierra and across much of the more remote western U.S.”

Environmentalists have expressed frustration over Gov. Jerry Brown’s “Wildfire Preparedness and Response” plan to craft policies to respond to increasing wildfire risk. The report blamed climate change but made no direct links to humans.

“There needs to be a shift to reducing the number of fire ignitions up front, rather than simply responding to fire damage — i.e., fastening your seatbelt vs. going to the hospital,” said David Kossack of the San Andreas Land Conservancy, a Davenport-based group which protects the forest along the San Mateo coast. Kossack is asking Insurance Commissioner Dave Jones to conduct an audit of fire sources.

“There is nothing ‘natural’ about any of these disasters,” he said.

Human-caused fires also are longer than lightning fires, Balch found. They last 85 days, on average; lightning fires last 45 days.

“When we have lightning events, smoke-jumpers are on them, right away. The U.S. Forest Service is pretty aggressive, and stops them when they are very, very small. They cut a ring around them, and then let the interior burn out. They might be accompanied by rain,” said Kurt Henke, former chief of the Sacramento Metro Fire Protection District.

“Human caused fires are closer to populated areas, with houses and lives at risk. We’ll stay on them until they’re completely out. And we’ll ‘mop up,’ digging up stumps, 200 feet out,” said Henke, now a consultant for major fire operations.

We’ve made trouble ever since we got here — yet education and regulations appear to be making a difference, Keeley found. We used to ignite a lot more fires early in the 20th century, when fire prevention was in its infancy and fire response was imperfect. The number of human-caused blazes increased steadily until 1979, then significantly declined with one big exception: power line ignitions. For example, the deadly fires last fall in Sonoma and Napa counties were sparked by downed power lines.

While electric power lines don’t account for many fires, they account for substantial amount of area burned. That’s because they commonly occur during high winds.

Historically, the largest number of fires were caused by equipment, such as gas-powered weed cutters that strike a rock and cause a spark, said Keeley. Ignitions also are triggered by generators, lawn mowers, chainsaws, tractors and off-road vehicles without required spark arrestors.

That is followed by arson, debris burning, kids playing with fire, smoking, vehicles and power lines.

The decline in human-sparked blazes can be attributed in part to better neighborhood watch programs, which include patrols during red flag warnings. There are increased penalties for arson. Debris burning is less accepted. Smoking, once a major cause of wildfires, has declined. Children have been taught not to play with fires, and lighters are childproof. Catalytic converters used to trigger fires when cars overheated; modern vehicles have warning lights when they overheat.

But catastrophic fires such as the Carr and Mendocino Complex — so early in the fire year — show how much more education and prevention is needed, said Henke.

“Data like this is critical, to help us analyze trends,” he said. “It tells us where we need to target.”

Lisa M. Krieger is a science writer at The Mercury News, covering research, scientific policy and environmental news from Stanford University, the University of California, NASA-Ames, U.S. Geological Survey and other Bay Area-based research facilities. Lisa also contributes to the Videography team. She graduated from Duke University with a degree in biology. Outside of work, she enjoys photography, backpacking, swimming and bird-watching.